I do a lot of riding in 100+ temps and under 15% humidity, and have crossed the Mojave many times during summer months. The cooling vests work wonderfully in these conditions, and will stay wet/damp for about 90 minutes, IF you restrict airflow a little, as others have said. I always wear a heavy jacket (not mesh), but one which has ARM ZIPPERS -- an absolute MUST for hot weather riding. Chest zips (IMHO) usually sit behind a windscreen, but arms/elbows are out in the breeze. Not sure why more manufacturers don't catch on to this trick.
Each to her/his own on using mesh riding gear, but if you use it, you must drink a lot of water to compensate for skin evaporation. Those guys who live in the Sahara and ride camels? No mesh gear there -- ATGATT, head-to-toe, and their lives depend on it, as do ours.
THE most important thing, as previously mentioned, is to STAY HYDRATED. In the west, this means a Camelbak or equivalent. You cannot stay hydrated in high heat and low humidity unless you are drinking small amounts every 15-30 minutes. You will get dehydrated if you rely on drinking at gas stops only, and many Western roads have 50+ mile stretches between gas, anyway.
The first thing that goes away when you get 10% dry is the "higher" thinking part, as in "making decisions," and you might find yourself trying to recall which lever is for stopping, and which for shifting. . .and why is my hand moving sooooo slow???